Libertarianism is suddenly in fashion. Denouncing the NSA, Rand Paul draws cheers both from young leftists in Berkeley and young conservatives in D.C.—and narrowly leads in early polls for the 2016 presidential nomination. The Koch brothers—who once bankrolled the Libertarian Party—plan to spend whatever it takes to elect anti-tax, anti-regulation Republicans. Same-sex marriage and the legalization of weed continue to gain support among the public and in the courts, while a majority of Americans recoil from a law that requires them to buy health insurance. Is a nation founded, in part, to defend individual freedom now ready to embrace politicians who will rigorously apply that principle to every significant matter of state?

Libertarianism may be on the rise, but it has no real chance of taking over the Republican Party, much less the nation. A daunting set of obstacles lies in the path of true believers who would shrink the government down to Gilded Age dimensions.

The most obvious hurdle is that Americans may dislike “big government,” but they cherish their federal benefits. The libertarian charge, made most recently by Paul Ryan, that entitlement programs harm the people they are supposed to help speaks to few recipients of Social Security or Medicare (even elderly Tea Partiers), much less to anyone cashing an unemployment check or being cared for at a VA hospital. And even most Republican businessmen would resist stripping away tax credits for homeowners and subsidies for energy and agriculture—just to name some of the biggest examples of “corporate welfare.”...